CDC Report: Vaccine effectiveness dropped from 91% to 66% since the SARS-CoV-2 Delta variant became predominant.

Effectiveness of COVID-19 Vaccines in Preventing SARS-CoV-2 Infection Among Frontline Workers Before and During B.1.617.2 (Delta) Variant Predominance — Eight U.S. Locations, December 2020–August 2021 – CDC Morbidity and Mortality Weekly Report

Commentaries:

Vaccines are less protective against Delta infection but still reduce risk by two-thirds, US CDC study shows – CNN

Messenger RNA COVID vaccines 66% effective against Delta: US study – Associated Press